There are many types and configurations of hearing losses that require interventions to help listeners function more effectively in the hearing world. Some listeners have losses that are successfully remediated with the use of appropriately fitted hearing aids. Others, especially those with severe and profound hearing losses, may not receive sufficient benefit from hearing aids to overcome the significant communication difficulties that result from the hearing loss. An option for some of these individuals may be a cochlear implant. Cochlear implants provide many potential benefits in both communication and quality of life. Since their introduction in the mid-1980s, the criteria for cochlear implant candidacy have expanded to include people with bilateral severe to profound hearing losses. Although the current criteria for candidacy varies slightly between manufacturers and devices, it includes those with a bilateral severe to profound loss who receive little or no benefit from hearing aids, as defined by poor speech recognition scores for sentences in the ear to be implanted as well as in the non-implanted ear or the binaural condition. Many individuals who experience significant communication difficulties due to their hearing loss, and report a high level of dissatisfaction from traditional hearing aids, may not meet the criteria for traditional cochlear implant candidacy. This is especially true for individuals with severe to profound hearing loss in the high frequencies who have significant residual low frequency hearing. These listeners typically experience significant difficulty understanding speech in real world listening environments even with state-of-the art, appropriately fitted hearing aids. However, their residual low frequency hearing disqualifies them from either meeting the FDA threshold and/or speech recognition criteria for traditional implantation. Additionally, traditional cochlear implant surgery carries the risk of damaging residual low frequency hearing in the implanted ear.
The recent advent of hybrid cochlear implant technology in the past five years may provide the ideal amplification solution for this population.
The Nucleus® Hybrid™ cochlear implant system offers individuals with steeply sloping hearing losses in the high-frequency region an alternative to traditional cochlear implantation. The Nucleus Hybrid cochlear implant system incorporates an internal device that has a shorter electrode array component than a more traditional cochlear implant. Like the traditional length cochlear implant, it uses electrical signals to stimulate the cochlea, but because of its shorter length, the Hybrid cochlear implant only stimulates the basal end of the cochlea where the highequency hearing has deteriorated, preserving the residual low-frequency hearing. This type of combination device, or Hybrid, can result in better hearing preservation than other traditional devices with longer arrays (Turner, Reiss, & Gantz, 2007). In addition, research has suggested that individuals with steeply sloping hearing losses may benefit more from short electrode implantation than longer electrode cochlear implants (Novak, Black, & Koch 2007;Gantz, Turner, Gfeller, & Lowder, 2005) because of the low-frequency hearing preservation.
The preservation of low frequency hearing has many advantages for cochlear implant users. Difficulty hearing in noise is a chief complaint of many people who use amplification, particularly those with greater degrees of hearing loss such as those using cochlear implants. Preserving low-frequency hearing can result in better understanding of speech in noise for cochlear implant users. Research indicates that preserved low-frequency hearing results in better frequency selectivity, which helps cochlear implant users understand speech in environments in the presence of background talkers (Turner, Reiss, & Gantz, 2007). It has been suggested that it is beneficial to preserve low-frequency hearing during cochlear implantation even if the individual's thresholds are in the severe range before implantation (Turner, Reiss, & Gantz, 2007). In addition to difficulty hearing in noise, a lack of music perception and appreciation is also an issue reported by many implant recipients. Because of the better frequency selectivity provided with a Hybrid device, short-electrode users may experience a heightened awareness and understanding of melodies, which can in turn give them an overall greater appreciation of music (Turner, Reiss, & Gantz, 2007;Gantz, Turner, & Gfeller, 2006;Gfeller, Olszewski, Turner, Gantz, Oleson, 2006;Gantz, Turner, Gfeller, & Lowder, 2005;Gantz & Turner, 2004;Gantz & Turner, 2003).
Over the past eight years, an FDA investigational research study sponsored by Cochlear Americas has been exploring the use of acoustic-electric stimulation to address the needs of individuals who demonstrate residual low-frequency hearing and severe to profound high-frequency sensorineural hearing losses (e.g., Luetje, Thedinger, Buckler, Dawson, & Lisbona, 2007;Gfeller et al., 2006;Gantz et al., 2005, 2006;James, Albegger, & Batmer, 2005;James et al., 2006;Gantz & Turner, 2004;Turner, Gantz, Vidal, Behrens, & Henry, 2004;Gantz & Turner, 2003). To date, three different electrode arrays have been designed by Cochlear Corporation to preserve residual hearing by using a shorter length cochlear implant: 1) the Nucleus® Hybrid™ S8 cochlear implant;2) the Nucleus® Hybrid™ S12 cochlear implant;and 3) the Nucleus® Hybrid™ L24 cochlear implant. Each of these electrode arrays is still under investigation through the Food and Drug Administration (FDA) and is undergoing clinical trials at this time. Cochlear Americas has sponsored Investigational Device Exemption (IDE) studies through the FDA to evaluate the safety and efficacy of these devices through multiple center studies. The remainder of this paper will discuss these cochlear implant options to preserve residual low-frequency hearing, preliminary results, and their candidacy criteria.
Nucleus® Hybrid™ S8
The Nucleus Hybrid S8 was designed to stimulate the high-frequency basal region of the cochlea, while preserving useful low-frequency acoustic hearing. This has been accomplished by employing a straight, intracochlear electrode array, which is connected to a Nucleus cochlear implant receiver/stimulator. The Hybrid S8 array is 10 mm in length, which is shorter than a conventional cochlear implant electrode array measuring 19 to 24 mm. The S8 device is also smaller in diameter (0.25 x 0.4 mm), and includes only six active electrodes and the standard two ground electrodes.
A total of 87 subjects have received the Nucleus Hybrid S8 cochlear implant in the United States to date. Speech perception results showed that the combined use of acoustic and electric stimulation has resulted in a significant improvement in speech perception in both quiet and in noise for the majority of these subjects (Gantz et al., 2005;Gantz & Turner, 2004;Turner et al., 2004;Gantz & Turner, 2003). Individuals received a 40% improvement in consonant recognition tasks with the addition of electric stimulation as compared to the acoustic only condition. In addition, some individuals improved their performance from postoperative consonant recognition scores ranging from 20 to 43% to postoperative scores in the 83 to 90% range, using the Hybrid S8 implant in the acoustic-plus-electric hearing conditions (Gantz & Turner, 2003).
Of the 87 subjects, preservation of some degree of residual low-frequency hearing in the implanted ear was successful in almost 85 of 87 (98%) immediately following implantation (within 1 month after surgery). This permitted the continued use of traditional amplification in the implanted ear for most subjects. Total loss of hearing occurred in 6 additional subjects between 3 and 24 months postactivation, resulting in some level of preservation of hearing in 79 of 87 (91%) subjects. At the subjects' most recent evaluations, ranging from 3 to36 months post-activation, 30% showed more than 30 dB mean low-frequency threshold shifts compared to pre-operative thresholds. (Gantz et al., in submission).
Nucleus® Hybrid™ S12
Earlier this year, another implant was introduced by Cochlear Corporation to stimulate the high-frequency basal region of the cochlea while at the same time preserving useful low-frequency acoustic hearing. This implant, the Nucleus Hybrid S12, is the same length as the Nucleus Hybrid S8 implant, but contains 10 active electrodes rather than the 6 active electrodes as described above. The purpose of the additional electrodes is to provide more detailed spectral information within the basal region of the cochlea. This electrode also contains two ground electrodes. Investigations are currently underway to assess the benefits with this electrode array.
Nucleus® Hybrid™ L24
A third electrode array designed to stimulate the high frequency region of the cochlea while preserving low frequency acoustic hearing is the Nucleus Hybrid L24 electrode array. This array is inserted 16 mm into the scala tympani instead of the 10 mm insertion as described with the Nucleus Hybrid S8 and S12 devices, and yet is still shorter than the 19-24 mm standard insertion of the Nucleus electrode array. It should be noted that the current Nucleus Contour Advance™ array, at 19 mm in length, is perimodiolar and therefore results in a deeper angular insertion depth than 19 mm would suggest. In addition, the L24 array has 22 active electrodes, the same as found on the standard-length Nucleus cochlear implants, rather than the 6 or 10 electrodes found on the S8 and S12 short arrays. The purpose of the L24 electrode array is to provide a similar number of contact sites as the standard length cochlear implant, but on a shorter length array. This design aims to potentially preserve acoustic low-frequency hearing while increasing the number of contact points along the modiolus for increased acoustic clarity.
Candidacy
Candidacy criteria for the S12 and L24 FDA Investigational Device studies have been developed. Recruitment for the S8 electrode array trial studies is no longer occurring;however the candidacy criteria were similar. The inclusionary and exclusionary criteria are listed below.
Inclusion criteria:
- Eighteen years of age or older at the time of implantation.
- Severe to profound sensorineural hearing loss for frequencies above 1500 Hz (average of 75 dB HL or greater at 2000 to 4000 Hz). Low-frequency thresholds up to and including 500 Hz should be no poorer than 60 dB HL.
- Consonant Nucleus Consonant (CNC) word recognition score between 10% and 60% in the ear to be implanted.
- CNC word recognition score in the contralateral ear equal to or better than the ear to be implanted, but not more than 80%.
- English spoken as a primary language.
- Duration of severe-to-profound hearing loss greater than 30 years.
- Congenital hearing loss.
- Medical or psychological conditions that contraindicate undergoing surgery.
- Ossification or any other cochlear anomaly that might prevent complete insertion of the electrode array.
- Conductive overlay of 15 dB or greater at two or more frequencies in the range 250 to 1000 Hz.
- Hearing loss of neural or central origin.
- Diagnosis of Auditory Neuropathy/Dys-synchrony.
- Active middle ear infection.
- Unrealistic expectations, on the part of the subject, regarding the possible benefits, risks, and limitations that are inherent to the surgical procedure(s) and prosthetic devices.
- Unwillingness or inability of the candidate to comply with all investigational requirements.
In order to evaluate a potential candidate's suitability for entrance into the studies based upon these exclusionary and inclusionary criteria, several steps need to be completed. Specifically, a history of hearing will be established by collecting information regarding the hearing etiology, onset of the hearing loss, duration of severe to profound hearing loss, and use of amplification. In addition, in order to determine that the subject is medically sound for cochlear implantation, the potential candidate's relevant medical and surgical history is discussed. Finally, subjects will be assessed using appropriately fitted behind-the-ear (BTE) or in-the-ear (ITE) hearing aids in each ear. Once the hearing aids have been fitted, unaided and aided audiometric thresholds, as well as unaided and aided speech discrimination assessments, will be obtained in order to determine if the subject meets the audiometric and speech discrimination requirements for inclusion into the study.
Once it has been determined that the subject meets all of the criteria for inclusion into the study, baseline auditory performance is established. This is done using a battery of speech perception and subjective self-report measures. The battery includes assessment of performance on word recognition in quiet and in adaptive noise levels, sentence recognition in fixed and adaptive noise levels, music perception, and self-report subjective assessments. The baseline testing will then serve useful in comparison to post-implant testing in order to analyze the potential benefits of a Nucleus Hybrid short electrode.
Post-Implant Testing
Following surgical implantation of the Hybrid cochlear implant, subjects are tested at numerous test sessions over the next year. The focus of the test session is to assess the stability of audiometric thresholds and also to evaluate speech perception performance. The battery of post-operative testing is similar to that of the pre-operative baseline testing. Testing is completed in a number of listening conditions, both in quiet and in noise, in order to assess the contribution of each of the different components of using a Hybrid cochlear implant. These conditions include:
- Hearing aid alone on the implanted side (ipsilateral hearing aid),
- Hybrid cochlear implant alone (cochlear implant only),
- Hybrid stimulation (cochlear implant and hearing aid on implanted side),
- Bimodal stimulation (implant + contralateral hearing aid),
- Combined stimulation (implant + ipsilateral and contralateral hearing aids).
The Nucleus Freedom for Hybrid sound processor was developed to provide acoustic and electrical stimulation to candidates that qualify for the Hybrid cochlear implant. It includes a processor similar to the behind-the-ear (BTE) commercially available Nucleus Freedom sound processor. However, this processor includes an acoustic component, called the Hybrid Acoustic Component (AC). The AC connects to the sound processor through a cable molded into the earhook of the speech processor. This serves to deliver acoustic amplification similarly to that of a conventional hearing aid (Figure 1).
Figure 1. Nucleus® Freedom™ Hybrid™ Sound Processor. Image courtesy of Cochlear Limited.
Programming of the sound processor is achieved via software, which allows programming of both the electric and acoustic parameters required for Nucleus Hybrid programming. The process of programming the electrical components is similar to that of programming traditional cochlear implants, except that the software provides more flexibility in setting the frequency boundaries assigned to the electric and acoustic domains of the Nucleus Hybrid cochlear implant. In addition, the software provides a user interface for the clinician to program the acoustical components, such as setting the gain and maximum output, frequency by frequency. The Nucleus Freedom for Hybrid sound processor is compatible with the Nucleus Hybrid S8 implant, the Nucleus Hybrid S12 implant, and the Nucleus Hybrid L24 implant.
Summary
The Nucleus Hybrid short electrode cochlear implant is a device with a shorter electrode array component than a traditional cochlear implant. Currently, three different arrays have been developed, each of which are implanted into the basal region of the cochlea. The goal of these arrays is to provide high-frequency stimulation to listeners with high-frequency hearing losses, while at the same time preserving residual low-frequency hearing. Preliminary results show that low-frequency hearing has been preserved for most listeners with this device. In addition, the majority of patients using the Nucleus Hybrid cochlear implant have experienced a significant improvement in speech perception in quiet and in noise and report a greater enjoyment of music. More studies are currently being conducted on the short electrode cochlear implant in order to further evaluate efficacy and safety of the device. Analyses are also being conducted to evaluate various demographic characteristics that may affect performance of the implant.
References
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